In a hot rolling process for manufacturing a hot strip, a slab heated to a high temperature is rolled so as to have a desired size and desired material properties, and then cooled with water on a run out table. The purpose of the water cooling is to obtain the desired material properties such as strength and ductility by mainly controlling the precipitates of the strip and the transformation structure of the strip. In particular, precisely controlling a cooling end temperature is significantly important to achieve the desired material properties without causing variation therefrom.
Although water, which is inexpensive, is often used as a cooling medium in a cooling step after hot rolling, use of such water cooling causes temperature variation of a strip at a low cooling end temperature or prevents precise stopping of the cooling at a desired temperature. The following factors are the main causes of these problems.
The first factor is a boiling state of water. In other words, cooling water boils when it comes into contact with a strip; however, the heat transfer performance of water changes at a certain temperature due to the conversion of the boiling state. When a hot strip is cooled lower than such a temperature, the cooling end temperature sometimes cannot be precisely controlled.
The boiling state of water in the case where a strip is cooled with water will now be described. Film boiling, nucleate boiling, and transition boiling occur, respectively, when the surface of a strip to be subjected to water is in a high temperature range, in a low temperature range, and in a middle temperature range between the high temperature range and the low temperature range. In the film boiling that occurs in a high temperature range, a vapor film is formed between the surface of a strip and cooling water. Since heat is transferred through thermal conduction within the vapor film, the cooling performance is low. In the nucleate boiling that occurs in a low temperature range, on the other hand, cooling water comes into direct contact with the surface of a strip and the cooling water is stirred due to the complicated phenomenon of the formation and disappearance of vapor bubbles, in which part of the cooling water vaporizes from the surface of a strip to form vapor bubbles and then the vapor bubbles are immediately condensed by the surrounding cooling water to disappear. As a result, significantly high cooling performance is achieved. In a middle temperature range, transition boiling in which the film boiling and the nucleate boiling coexist occurs. Unlike the nucleate boiling and the film boiling, heat flux increases as strip temperature decreases in this transition boiling. In terms of controlling material properties, the cooling rate should not vary with temperature. Furthermore, when cooling is stopped (ended) in a temperature range where the conversion from film boiling to transition boiling occurs, even a slightly long control time for cooling causes a problem in that the strip temperature is considerably lowered from a desired temperature because the cooling rate increases at an accelerated pace in a transition boiling temperature range.
In the case where a strip before cooling has locally low-temperature regions due to, for example, hot rolling, the transition boiling occurs first in these low-temperature regions during cooling, which causes further temperature deviation. In a cooling step conducted on a general run out table, such transition boiling begins at about 500° C.
The second factor is residual cooling water on a strip. Although laminar cooling is conducted using round or slit type nozzles when the upper side of a strip is cooled on a general run out table, cooling water that collides with the upper side of the strip flows in a direction of movement of the strip while being left on the strip. Normally, cooling water on the upper side of the strip is drained by being purged. In a conventional method, however, cooling water is purged at a position distant from the place where the cooling water is supplied. Therefore, only the part where the cooling water is left on the surface of the strip is excessively cooled before the cooling water is purged. In particular, in the low temperature range of 500° C. or less, the cooling performance of the cooling water becomes high due to the conversion of the boiling state from the film boiling to the transition boiling, resulting in a large temperature deviation between the regions where residual cooling water exists and the regions where residual cooling water does not exist.
From the reasons described above, the temperature in a coil is significantly varied when cooling of a hot strip is stopped at 500° C. or less, which is a transition boiling initiation temperature. Thus, various methods have been examined to deal with the above-mentioned phenomena.
For example, a method for supplying cooling water to both the upper and lower sides of a hot strip in the high temperature range where film boiling occurs, and supplying the cooling water to only the lower side of the strip in the temperature range of transition boiling is disclosed in Patent Document 1. In this cooling method, residual cooling water on the upper side of the strip and the thermal instability in cooling caused by the residual cooling water are removed by cooling only the lower side in the temperature range of transition boiling, to realize stable cooling.
A method for conducting cooling using low-temperature cooling water first, and then conducting cooling using cooling water having a high temperature of 80° C. or more from the temperature range of transition boiling is disclosed in Patent Document 2. In this cooling method, the transition boiling initiation temperature is shifted to the low temperature side by using hot water as cooling water, thereby lengthening the duration of film boiling, to realize stable cooling.
A method for disposing a water cooling apparatus together with a gas cooling apparatus, conducting water cooling with the water cooling apparatus in a high temperature range, and conducting gas cooling with the gas cooling apparatus in a temperature range that is lower than the transition boiling initiation temperature is disclosed in Patent Document 3. In this cooling method, the gas cooling that does not cause a boiling phenomenon and shows a stable cooling performance in the low temperature range is used to realize temperature stability in the low temperature range.
A method for conducting cooling to about 400° C. with hot water of 80 to 100° C. in the first half of a run out table, and then conducting cooling with cooling water having a temperature lower than that used in the first half of a run out table is disclosed in Patent Document 4. In this cooling method, the transition boiling initiation temperature is shifted to the low temperature side by using hot water as cooling water in the first half of a run out table, and cooling is conducted with cooling water that is cold enough to cause nucleate boiling in the low temperature range, to realize temperature stability in the low temperature range.
The following cooling apparatus is disclosed in Patent Document 5. In this cooling apparatus, a cooling zone where cooling water is supplied to continuously cool a strip after hot finishing rolling is divided into a first zone and a second zone. A cooling device with a high cooling performance (water flow rate: 1.0 to 5.0 m3/m2·min) is disposed in the first zone and a cooling device with a low cooling performance (water flow rate: 0.05 m3/m2·min to less than 0.3 m3/m2·min) is disposed in the second zone. In addition, a cooling device with a middle cooling performance (water flow rate: 0.3 m3/m2·min to less than 1.0 m3/m2·min) is disposed throughout the cooling zone. In the cooling of a hot strip with such a cooling apparatus, the transition boiling initiation temperature is shifted to the low temperature side by decreasing the amount of cooling water in the low temperature range, thereby lengthening the duration of film boiling to realize stable cooling.    Patent Document 1: Japanese Examined Patent Application Publication No. 6-248    Patent Document 2: Japanese Unexamined Patent Application Publication No. 6-71339    Patent Document 3: Japanese Unexamined Patent Application Publication No. 2000-313920    Patent Document 4: Japanese Unexamined Patent Application Publication No. 58-71339    Patent Document 5: Japanese Unexamined Patent Application Publication No. 2003-25009